Communication cables comprised of multiple twisted pairs of conductors are common, with four-pair cables being widely used. In high-speed data networks, crosstalk can result within communication cables and between nearby communication cables. Crosstalk occurring within a cable includes near-end crosstalk (NEXT) and far-end crosstalk (FEXT), and alien crosstalk occurring between cables includes alien near-end crosstalk (ANEXT) and alien far-end crosstalk (AFEXT). Suppression of alien crosstalk in communication channels is important, because alien crosstalk can reduce the signal-to-noise ratio in a communication channel and increase the channel's bit error rate. As communication bandwidth increases, the reduction of noise such as alien crosstalk in communication cables becomes increasingly important.
In high-bandwidth communication applications, communication cables are commonly installed alongside one another, and ANEXT and AFEXT can result between adjacent or nearby communication cables. ANEXT and AFEXT become more problematic at frequencies above 10 MHz, and ANEXT and AFEXT noise at high frequencies are present in high-speed data transmission systems such as 10 Gigabit Ethernet signaling.
Alien crosstalk includes the following:
1. Differential mode crosstalk produced by differential signals propagating in a twisted pair in one cable coupling to another twisted pair in another cable;
2. Common mode crosstalk produced by common mode signals propagating in one cable or external sources coupling to all wires in another cable. This coupled common mode signal can then convert to differential mode alien crosstalk. The conversion is typically produced by a wire pair imbalance or a connecting hardware imbalance.
3. Differential mode crosstalk produced by a differential signal propagating between two twisted pairs in one cable coupling to the wires in another cable either differentially or in common mode. This differential signal propagating via two twisted pairs in a cable is also called a “super pair mode,” which can be produced in connecting hardware due to a “split pair” (wires 3 and 6) coupling to wire pairs 1-2 and 7-8 forming a “super” twisted pair.
ANEXT and AFEXT arise due to electrical and magnetic couplings between conductors in different cables. The magnitude of ANEXT in twisted pair systems is proportional to the difference between the magnitude of the electrical coupling and the magnitude of the magnetic coupling (in the following formulas, “C” refers to coupling):|ANEXT|=|C(electric)−C(magnetic)|.
In order to decrease ANEXT, the electrical and magnetic couplings can both be decreased. For example, assume that the C(electric)=Ce=0.25 and C(magnetic)=Cm=0.15, then the difference, Cd=0.1. If both couplings are reduced by an order of magnitude, then Ce=0.025 and Cm=0.015 and the difference would be Cd=0.01. NEXT can also be reduced by reducing the coupling giving rise to the larger of the two magnitudes. For example, again assuming that Ce=0.25 and Cm=0.15 corresponding to a Cd=0.1. If Ce is reduced by 20% or Ce=0.2 then Cd will be reduced to Cd=0.05.
AFEXT in twisted pair systems is found by determining the sum of the electrical coupling and the magnetic coupling:|AFEXT|=|C(electric)+C(magnetic)|.In order to decrease AFEXT, either or both of the electrical coupling and the magnetic coupling should be reduced.
It is desirable to reduce alien crosstalk. It is particularly desirable to achieve this reduction in a way that addresses the electrical and magnetic couplings that give rise to alien crosstalk.